1. Field of Invention
This invention pertains generally to tuning forks for use in rate sensors and, more particularly, to a tuning fork and method in which mass balance is maintained when quadrature error is reduced.
2. Related Art
Tuning fork rate sensors make use of the Coriolis effect to sense rotation. The tuning fork is driven to oscillate in a drive move in which the tines oscillate in a plane with roughly equal and opposite amplitudes. Under rotation, the tines experience a Coriolis acceleration proportional to the velocity of the tines and in a direction orthogonal to the drive motion. In a double-ended tuning fork, the orthogonal acceleration excites a pickup mode of vibration which causes both the driven set of tines and the other (pickup) set to vibrate out of the plane of the device. In quartz rate sensors, this out-of-plane vibration is detected piezoelectrically in a manner well known in the art.
A common problem in such devices is that minute variations in fabrication of the tuning fork can cause relatively large errors in the output. These fabrication errors can arise due to faceting in crystalline materials and/or errors in the cross-sectional geometry of the tines (i.e., the tines are not perfectly rectangular). In either case, the drive tines do not oscillate in precisely the same plane, and this creates a torsional couple about the axis of symmetry of the tines. The net effect of this torsional couple is to cause the tines to oscillate in the pickup mode of vibration with a phase which is shifted by 90 degrees relative to the desired rotation signal. The shifted signal is commonly referred to as a quadrature error signal.
The quadrature error signals are often much larger than the desired full scale output of the sensor. It is, therefore, necessary to find a way to reduce these error signals to make practical devices. This is conventionally done by plating masses of material (typically gold) on the ends of the tines and then selectively removing the material from one of the tines with a laser. When mass is removed in this manner, the tines are subjected to a torsional twist about their long axes. If these torsional twists are different in the two tines, a net coupling into the pickup mode oscillation occurs, and this coupled signal can be used to negate the quadrature signals caused by fabrication errors.
The use of a laser for trimming mass in this manner is advantageous in that it is a readily automated and capable of high rates of production. However, it also has a significant disadvantage in that the two tines become unbalanced in mass. This imbalance causes the moments of the tines to be unequal so that a net lateral force is transmitted to the base of the tines and, ultimately, to the fork mount and its surroundings. This coupling of the tuning fork to the surrounding environment degrades the performance of the sensor, particularly if this environment includes any elements with mechanical resonances at or near the drive frequency of the fork.
U.S. Pat. No. 3,683,213 discloses a single ended tuning fork or microresonator which is intended primarily for use as a time standard in wrist watches. In this device, metal film weights are placed on the top surfaces of the tines near their free ends and trimmed to adjust the frequency of the microresonator. In this particular example, the trimming is done only for purposes of frequency adjustment, and has no relevance to quadrature reduction or tine balance.
U.S. Pat. No. 4,379,244 discloses a tuning fork which has electrodes near the stem of the fork for detecting a voltage which is indicative of asymmetrical oscillation of the tines. A laser is used for removing mass from the front surfaces of the tines in order to provide a symmetrical oscillation of the tines and thus a balanced condition. While this technique may result in a balanced fork, it is not useful in tuning fork rate sensors because it does not provide any adjustment of the quadrature output, and the quadrature offset would, in general, remain quite large.